list: list_append_list / list_prepend_list

Operations for moving the entire contents of a list.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

ccan/list/list.h

@@ -371,6 +371,68 @@ static inline const void *list_tail_(const struct list_head *h, size_t off)
 #define list_for_each_safe(h, i, nxt, member)				\
 	list_for_each_safe_off(h, i, nxt, list_off_var_(i, member))
 
+/**
+ * list_append_list - empty one list onto the end of another.
+ * @to: the list to append into
+ * @from: the list to empty.
+ *
+ * This takes the entire contents of @from and moves it to the end of
+ * @to.  After this @from will be empty.
+ *
+ * Example:
+ *	struct list_head adopter;
+ *
+ *	list_append_list(&adopter, &parent->children);
+ *	assert(list_empty(&parent->children));
+ *	parent->num_children = 0;
+ */
+static inline void list_append_list(struct list_head *to,
+				    struct list_head *from)
+{
+	struct list_node *from_tail = list_debug(from)->n.prev;
+	struct list_node *to_tail = list_debug(to)->n.prev;
+
+	/* Sew in head and entire list. */
+	to->n.prev = from_tail;
+	from_tail->next = &to->n;
+	to_tail->next = &from->n;
+	from->n.prev = to_tail;
+
+	/* Now remove head. */
+	list_del(&from->n);
+	list_head_init(from);
+}
+
+/**
+ * list_prepend_list - empty one list into the start of another.
+ * @to: the list to prepend into
+ * @from: the list to empty.
+ *
+ * This takes the entire contents of @from and moves it to the start
+ * of @to.  After this @from will be empty.
+ *
+ * Example:
+ *	list_prepend_list(&adopter, &parent->children);
+ *	assert(list_empty(&parent->children));
+ *	parent->num_children = 0;
+ */
+static inline void list_prepend_list(struct list_head *to,
+				     struct list_head *from)
+{
+	struct list_node *from_tail = list_debug(from)->n.prev;
+	struct list_node *to_head = list_debug(to)->n.next;
+
+	/* Sew in head and entire list. */
+	to->n.next = &from->n;
+	from->n.prev = &to->n;
+	to_head->prev = from_tail;
+	from_tail->next = to_head;
+
+	/* Now remove head. */
+	list_del(&from->n);
+	list_head_init(from);
+}
+
 /**
  * list_for_each_off - iterate through a list of memory regions.
  * @h: the list_head

ccan/list/test/run-prepend_list.c

+#include <ccan/list/list.h>
+#include <ccan/tap/tap.h>
+#include <ccan/list/list.c>
+#include <stdarg.h>
+
+static bool list_expect(struct list_head *h, ...)
+{
+	va_list ap;
+	struct list_node *n = &h->n, *expected;
+
+	va_start(ap, h);
+	while ((expected = va_arg(ap, struct list_node *)) != NULL) {
+		n = n->next;
+		if (n != expected)
+			return false;
+	}
+	return (n->next == &h->n);
+}
+
+int main(int argc, char *argv[])
+{
+	struct list_head h1, h2;
+	struct list_node n[4];
+
+	plan_tests(40);
+
+	list_head_init(&h1);
+	list_head_init(&h2);
+
+	/* Append an empty list to an empty list. */
+	list_append_list(&h1, &h2);
+	ok1(list_empty(&h1));
+	ok1(list_empty(&h2));
+	ok1(list_check(&h1, NULL));
+	ok1(list_check(&h2, NULL));
+
+	/* Prepend an empty list to an empty list. */
+	list_prepend_list(&h1, &h2);
+	ok1(list_empty(&h1));
+	ok1(list_empty(&h2));
+	ok1(list_check(&h1, NULL));
+	ok1(list_check(&h2, NULL));
+
+	/* Append an empty list to a non-empty list */
+	list_add(&h1, &n[0]);
+	list_append_list(&h1, &h2);
+	ok1(list_empty(&h2));
+	ok1(list_check(&h1, NULL));
+	ok1(list_check(&h2, NULL));
+	ok1(list_expect(&h1, &n[0], NULL));
+
+	/* Prepend an empty list to a non-empty list */
+	list_prepend_list(&h1, &h2);
+	ok1(list_empty(&h2));
+	ok1(list_check(&h1, NULL));
+	ok1(list_check(&h2, NULL));
+	ok1(list_expect(&h1, &n[0], NULL));
+
+	/* Append a non-empty list to an empty list. */
+	list_append_list(&h2, &h1);
+	ok1(list_empty(&h1));
+	ok1(list_check(&h1, NULL));
+	ok1(list_check(&h2, NULL));
+	ok1(list_expect(&h2, &n[0], NULL));
+
+	/* Prepend a non-empty list to an empty list. */
+	list_prepend_list(&h1, &h2);
+	ok1(list_empty(&h2));
+	ok1(list_check(&h1, NULL));
+	ok1(list_check(&h2, NULL));
+	ok1(list_expect(&h1, &n[0], NULL));
+
+	/* Prepend a non-empty list to non-empty list. */
+	list_add(&h2, &n[1]);
+	list_prepend_list(&h1, &h2);
+	ok1(list_empty(&h2));
+	ok1(list_check(&h1, NULL));
+	ok1(list_check(&h2, NULL));
+	ok1(list_expect(&h1, &n[1], &n[0], NULL));
+
+	/* Append a non-empty list to non-empty list. */
+	list_add(&h2, &n[2]);
+	list_append_list(&h1, &h2);
+	ok1(list_empty(&h2));
+	ok1(list_check(&h1, NULL));
+	ok1(list_check(&h2, NULL));
+	ok1(list_expect(&h1, &n[1], &n[0], &n[2], NULL));
+
+	/* Prepend a 2-entry list to a 2-entry list. */
+	list_del_from(&h1, &n[2]);
+	list_add(&h2, &n[2]);
+	list_add_tail(&h2, &n[3]);
+	list_prepend_list(&h1, &h2);
+	ok1(list_empty(&h2));
+	ok1(list_check(&h1, NULL));
+	ok1(list_check(&h2, NULL));
+	ok1(list_expect(&h1, &n[2], &n[3], &n[1], &n[0], NULL));
+
+	/* Append a 2-entry list to a 2-entry list. */
+	list_del_from(&h1, &n[2]);
+	list_del_from(&h1, &n[3]);
+	list_add(&h2, &n[2]);
+	list_add_tail(&h2, &n[3]);
+	list_append_list(&h1, &h2);
+	ok1(list_empty(&h2));
+	ok1(list_check(&h1, NULL));
+	ok1(list_check(&h2, NULL));
+	ok1(list_expect(&h1, &n[1], &n[0], &n[2], &n[3], NULL));
+
+	return exit_status();
+}